The present invention relates to pressure transducers and compositions for making pressure transducers and in particular to transducers having a variable resistance layer made from a composition of stannous oxide particulate intermixed with a continuous matrix conductive resin which is applied in a thin layer to a surface and thereafter cured to form the resistance layer.
Pressure responsive transducers are known. For example, U.S. Pat. No. 3,806,471 to Mitchell describes a typical pressure transducer wherein conduction paths through a volume of a thick compressible layer would be increased thereby decreasing the resistance through the layer in response to application of an external force. These volume type pressure transducers were subject to many disadvantages including the size of the transducer and the relative rapid degeneration in performance.
An improved pressure transducer relying on variable contact across the surface of a thin layer is disclosed in Eventoff U.S. Pat. Nos. 4,489,302 and 4,314,227. While the Eventoff transducer provided significant improvement in pressure transducers, problems still remained. For example, performance characteristics varied widely depending on the characteristics of the composition and the care with which the manufacturing process was carried out. Even under stringent process controls, performance varied widely requiring that each transducer be tested in advance of use to determine if the transducer would meet specifications in those applications for which predictable performance was required
Although in many applications the need to maintain precise performance specifications was unnecessary and hence wide variations in performance were tolerable, many applications required precise, repeatable and predictable performance. Furthermore, certain applications required performance characteristics to remain the same regardless of environmental factors such as heat and humidity.
In order to solve the problems with prior resistance plies and compositions and achieve the above advantages, numerous attempts over a period of more than seven years were made using different particulate materials and particulate and resin compositions. However, the performance deficiencies set forth above were not solved. Accordingly, the need for a pressure transducer which not only would be largely immune to manufacturing variations so as to be able to manufacture transducers with repeatable and predictable performance characteristics remained. Further, it was desired to have a transducer which would continue to perform according to a predefined specification despite variations in temperature and humidity. Finally, it was desired to have a composition for making the resistance layer forming the essential component of the pressure transducer which would have an extended shelf life.
Each of these advantages and objectives is met by the pressure transducer made with a composition comprising a mixture of a conductive resin and stannous oxide particulate. The conductive resin is optionally made of a carbon filled resin mixed with stannous oxide particulate preferably in a ratio of 1 part conductive resin to 4 parts stannous oxide. The resultant composition has a shelf life which is virtually unlimited. Furthermore, the conductive resin and stannous oxide particulate composition has been determined to be largely immune from manufacturing conditions which substantially affected the performance of prior transducers having resistance layers made with other particulate such as molybdenum disulfide or silica/alumina.
The present invention results in a pressure transducer which has stable, predictable performance characteristics largely independent of manufacturing conditions and subsequent environmental changes permitting a significant reduction in the cost of manufacture and an increased cost effective means of meeting customer tolerances and specifications. Further, the composition used to make the resistance layer has a long shelf life which enabled greater flexibility in the manufacture of force transducers.